The cause for the changing appearance of the Moon—its phases—is a difficult concept for many people to learn, and this gives rise to some surprisingly stubborn preconceptions. The phases are caused by the fact that we can only see the part of the Moon that is illuminated by the Sun. The amount of the illuminated surface that we can see from Earth varies as the Moon orbits Earth.

This Reading provides background information on the underlying concepts in the Activities included in the book, Project Earth Science: Astronomy, Revised 2nd Edition. The Readings go beyond what is found in the background information in the Activities. They can be used as supplementary information for the teacher, or to enhance classroom discussions.